Construction began on the 1.3km long fourth bore of the Caldecott Tunnel last month. The four year US$420 million project will run between Oakland and Contra Costa County on Highway 24 and is aimed at alleviating traffic congestion in the area.

Tutor-Saliba is constructing

the two-lane 12.5m wide

bore for a partnership between the California Department of Transportation (CALTRANS),

the Contra Costa Transportation Authority (CCTA) and the Alameda County Congestion Management Agency (ACCMA).

It was awarded the contract

last September.

Excavation from both ends is expected to start in June. It will be constructed using NATM. Roadheaders are expected to

be used for most of the excavation but some drill and blast might be performed through harder rock. One to two metres per day will be excavated. The support for the tunnel will primarily include rock bolts, lattice girders and shotcrete.

A spokesman for the project told T&TI that one of the main challenges in construction will be disposing of the spoil as a site for this has not been found.

“Blasting may also be an issue. The limitations on blasting are quite restrictive; no night blasting. Daytime blasting requires the closure of the third bore, which according to the traffic plan, can only be closed for a short period of time near midday. Of course, other operation must be halted before blasting can occur,” the spokesman told T&TI.

Seven cross-passages to the third bore will be built to provide emergency exits.

Work is scheduled for completion in spring 2014 and will eliminate the need for the existing central tunnel to alternate between westbound and eastbound traffic during rush hour.

Orinda councilor Amy Rein Worth who fought for the project for years before seeing it get the go-ahead said, “Given the decades it took to put this project together, four years will seem like a very short time.”

Bores one and two were opened in 1937 and the third added in 1964. Some 160,000 commuters use the tunnel daily.